$items = array(
array('id'=>1,'pid'=>0,'name'=>'一級11'),
array('id'=>2,'pid'=>0,'name'=>'一級12'),
array('id'=>3,'pid'=>1,'name'=>'二級21'),
array('id'=>4,'pid'=>3,'name'=>'三級31'),
array('id'=>5,'pid'=>1,'name'=>'二級22'),
array('id'=>6,'pid'=>3,'name'=>'三級32'),
array('id'=>7,'pid'=>6,'name'=>'四級41'),
);
$i = 0;
function formatTree($arr, $pid = 0){
$tree = array();
$temp = array();
global $i;
if($arr){
foreach($arr as $k=>$v){
if($v['pid'] == $pid){//
$temp = formatTree($arr, $v['id']);
$temp && $v['son'] = $temp;
$tree[] = $v;
}
}
}
return $tree;
}
print_r(formatTree($items));

function genTree($items) {
    $tree = array(); //格式化好的樹
    foreach ($items as $item)
        if (isset($items[$item['pid']])){
            $items[$item['pid']]['son'][] = &$items[$item['id']];
        }
        else{
            $tree[] = &$items[$item['id']];
        }
    return $tree;
}

$items = array(
    1 => array('id' => 1, 'pid' => 0, 'name' => '一級11'),
    2 => array('id' => 2, 'pid' => 1, 'name' => '二級21'),
    3 => array('id' => 3, 'pid' => 1, 'name' => '二級23'),
    4 => array('id' => 4, 'pid' => 9, 'name' => '三級31'),
    5 => array('id' => 5, 'pid' => 4, 'name' => '四級43'),
    6 => array('id' => 6, 'pid' => 9, 'name' => '三級32'),
    7 => array('id' => 7, 'pid' => 4, 'name' => '四級41'),
    8 => array('id' => 8, 'pid' => 4, 'name' => '四級42'),
    9 => array('id' => 9, 'pid' => 1, 'name' => '二級25'),
    10 => array('id' => 10, 'pid' => 11, 'name' => '二級22'),
    11 => array('id' => 11, 'pid' => 0, 'name' => '一級12'),
    12 => array('id' => 12, 'pid' => 11, 'name' => '二級24'),
    13 => array('id' => 13, 'pid' => 4, 'name' => '四級44'),
    14 => array('id' => 14, 'pid' => 1, 'name' => '二級26'),
    15 => array('id' => 15, 'pid' => 8, 'name' => '五級51'),
    16 => array('id' => 16, 'pid' => 8, 'name' => '五級52'),
    17 => array('id' => 17, 'pid' => 8, 'name' => '五級53'),
    18 => array('id' => 18, 'pid' => 16, 'name' => '六級64'),
);
print_r(genTree($items));